Particularly in undersea pipeline installations, it is highly desirable to be able to remove a pipeline valve, or its internal components, for purposes of repairing and replacing same while continuing to support the pipeline during the removal and replacement operation.
Some have suggested the provision of top-entry valves wherein access to the valve internal components is gained by removal of a top closure. However, the repair of such valves requires a number of mechanical manipulations such as the removal and replacement of bolts, the main valve member and the seat ring assemblies, and such operations are not ideally performed on the ocean floor, where maneuverability and mechanical leverage are impaired and visibility is generally poor. Moreover, a part dropped in the silt and vegetation of the ocean floor may not be easily found.
There are commercially available valves of the expansible tube type, wherein a cartridge containing all of the operating parts of the valve is clamped between two line flanges which, in turn, are interconnected by threaded studs. In order to remove the cartridge from between the line flanges, the studs across the top are removed and the nuts on the remaining studs are loosened. Jacking nuts are provided on at least some of the remaining studs on the inboard side of the flanges so that they can be threaded outward to jack the line flanges apart for removal of the valve cartridge. However, in undersea operations, the seas, tides and shifting sands impose many additional forces on the pipeline which, with the removal of studs between the line flanges could impose severe stresses on the remaining studs and prevent proper realignment when the valve cartridge is replaced. Moreover, the studs and nuts removed from the valve for purposes of valve cartridge replacement, are easily lost in the silt, sands and vegetation on the ocean floor where visibility is something less than ideal.